[FLINK-36428][Connectors/DynamoDB] DynamoDb Table API Sink fails when null value in the RowData

[maoxingda]

Purpose of the change

Add an option ignore-nulls to determine whether to ignore null values, instead of hardcoding it into the code.

Verifying this change

Please make sure both new and modified tests in this PR follows the conventions defined in our code quality guide: https://flink.apache.org/contributing/code-style-and-quality-common.html#testing

(Please pick either of the following options)

This change is a trivial rework / code cleanup without any test coverage.

(or)

This change is already covered by existing tests, such as (please describe tests).

(or)

This change added tests and can be verified as follows:

(example:)

• Added integration tests for end-to-end deployment
• Added unit tests
• Manually verified by running the Kinesis connector on a local Flink cluster.

Significant changes

(Please check any boxes [x] if the answer is "yes". You can first publish the PR and check them afterwards, for convenience.)

• [ ] Dependencies have been added or upgraded
• [ ] Public API has been changed (Public API is any class annotated with @Public(Evolving))
• [ ] Serializers have been changed
• [ ] New feature has been introduced
  • If yes, how is this documented? (not applicable / docs / JavaDocs / not documented)